Recent years have witnessed a great increase of tablet electronic devices such as a portable phone, a personal computer and a music player, each of which is equipped with a touch panel. Such electronic devices display various operation buttons on their display screens. This allows a user to carry out an input operation to the electronic devices in a dialogue form by touching a desired operation button with the user's finger, or a pen or the like, instead of using a conventional user interface such as a keyboard or a button.
A capacitive touch panel, which is a touch panel of a capacitive type that is one of operation types of touch panels, operates by detecting a change which is caused in a sensor by a capacitance of a human finger. One of main noises which occur during sensing of the capacitive touch panel is a driving noise of a liquid crystal module (hereinafter also referred to as a liquid crystal driving noise), which is provided in the vicinity of a back side of the capacitive touch panel. In a case where the liquid crystal driving noise is mixed into the sensing, a decrease in recognition performance of a touch panel is caused. Therefore, it has been an object to carry out sensing (scanning) which enables decreasing or avoiding a liquid crystal driving noise.
In order to avoid the mixing of the liquid crystal driving noise into sensing, in one method, a frequency of the liquid crystal driving noise and a noise frequency which is sensed by the touch panel as a noise are set so as not to overlap each other. In a case where the frequency of the liquid crystal driving noise and the noise frequency sensed by the touch panel do not overlap each other, it is possible to prevent the liquid crystal driving noise from mixing into the sensing of the touch panel. This alleviates a decrease in recognition performance of the touch panel.
A conventional liquid crystal display device is schematically illustrated in FIG. 9. As illustrated in FIG. 9, in a conventional liquid crystal display device 30, a liquid crystal driving controller (timing generator) 5 is provided with an oscillation circuit 7a which supplies a reference clock signal CLK to the liquid crystal driving controller (timing generator) 5, whereas a touch panel controller 4 of a capacitive touch panel 2 is provided with an oscillation circuit 7b which supplies a reference clock signal CLK to the touch panel controller 4. A liquid crystal panel 3 is driven in accordance with the reference clock signal CLK supplied to the timing generator 5. Similarly, the touch panel 2 carries out sensing in accordance with the reference clock signal CLK supplied to the touch panel controller 4.
A frequency of each of the reference clock signals CLK has a variation of about ±a few % to ±10%, as a device characteristic which is caused by an environmental change such as a change in temperature, or as an individual difference from an individual variation or the like. As such, in a case where there is a variation in the frequency of each of the reference clock signals CLK, there is a variation in a frequency at which a liquid crystal is driven (hereinafter also referred to as a “liquid crystal driving frequency”) and in a frequency at which a touch panel is scanned (hereinafter also referred to as a “scanning frequency”). This makes it difficult to prevent the frequency of the liquid crystal driving noise and the noise frequency sensed by the touch panel from overlapping each other. This increases a possibility that the liquid crystal driving noise is mixed into the sensing of the touch panel 2 (arrow N in FIG. 9).
In order to decrease or avoid the liquid crystal driving noise, in one method, a gap between a source of the liquid crystal driving noise and a touch panel (sensor) is extended, or in another method, a shield layer is added. However, since display devices are becoming thinner nowadays, it is difficult to extend the gap between the touch panel and a liquid crystal panel, which is the source of the liquid crystal driving noise. Furthermore, newly providing the shield layer may increase costs.
In view of the problems, for example, Patent Literature 1 discloses a configuration in which liquid crystal driving is synchronized with sensing of a touch panel, so that the sensing is carried out in a blanking period of a liquid crystal. Patent Literature 1 also discloses a configuration in which, in carrying out the sensing in the blanking period, an active period is shorten in order to have a longer blanking period.
According to the above configurations, it is possible to decrease an effect of the liquid crystal driving noise since the sensing is carried out in the blanking period during which a data driver and a gate driver of a liquid crystal display device are not driven. Therefore, it is possible to prevent a decrease in recognition performance of the sensing.